Conventionally, a rotation angle detector detects a rotation angle by using a magnetic detector at a periphery of a rotating magnet (e.g., a permanent magnet, a permanent magnet with a yoke or the like) that generates magnetic flux.
FIGS. 13A and 13B shows a rotation angle detector that includes a shaft 1, a cylindrical magnet 3 attached to the shaft 1, and a magnetic detector 4 disposed at a rotation center of the magnet 3 for detecting magnetic flux generated by the magnet 3. The magnet 3 is an example of a magnetic flux generation means, and the magnetic detector 4 includes two magnetic sensors, that is, a first magnetic sensor 6 and a second magnetic sensor 7 for detecting a wide range of rotation angle. The first and the second magnetic sensors 6, 7 detect the magnetic flux in a plane that is perpendicular to the shaft 1, and the sensors 6, 7 are used for detecting an angle of the magnetic flux at an angular interval of 90 degrees and for outputting a detection signal.
The magnetic sensors 6, 7 in the magnetic detector 4 output the detection signal relative to the magnetic flux in a shape of sine curve and in a shape of cosine curve, and the detection signals are converted by an inverse-trigonometric calculation unit (a microcomputer) to have a linear characteristic of increasing nature in an interval of 180 degrees. The detection signals converted in this manner are connected for a coverage of 360 degrees. This method of rotation angle detection is disclosed in a Japanese Patent Document JP-A-2003-75108, and diagrams shown in FIGS. 3A, 3B and 3C illustrates the sine/cosine curves and converted linear outputs from the detector 4.
However, the magnetic flux generated by the cylindrical magnet 3 is warped around the shaft 1 under the influence of magnetism of the shaft 1 when the rotation angle detector has the magnetic detector 4 displaced away from the rotation center of the cylindrical magnet 3 as shown in FIGS. 14A and 14B. Therefore, the rotation angle cannot be correctly detected when the magnetic flux is warped in a space surrounded by the cylindrical magnet 3.
In an attempt to avoid warpage of the magnetic flux, the rotation angle detector shown in FIGS. 15A and 15B has a magnet having a disk shape.
In this case, the magnetic flux is still being warped at a trace line α in FIG. 16 under the influence of the shaft 1. The warpage of the direction of the magnetic flux is represented by arrows around a point β in FIG. 16. As a result, the detection signals from the sensors 6, 7 having magnetism sensing surfaces aligned with a tangential line and a normal line of the trace line (a circle) α do not have matching amplitude, thereby increasing the amount of detection error G as shown in FIG. 17.
Further, the magnetic flux in a circle γ forms an open magnetic field at a proximity of the trace line α of the magnetic detector 4 as shown in FIG. 18. That is, the direction of the magnetic flux is not uniformly aligned to a certain direction, thereby increasing errors in the detection signal caused by a small displacement of the magnetic sensors or the like. As a result, a nominal assembly displacement of the first or second sensors 6, 7 suffers from an increased amount of warpage in the magnetic flux as the error in the detection signal.